Outage margin and power constraints in cognitive radio with multiple antennas

In the commons model for spectrum usage, the cognitive (secondary) users are allowed to use the spectrum as long as the target performance in the primary system is not violated. In this paper we consider primary system that has a target outage performance and the transmission power of the secondary transmitter (STX) should be appropriately not to violate the target outage probability for the primary terminals (PT). We have considered two types of STX, with single antenna (SISO STX) and two antennas (MISO STX) and analyzed the allowed secondary power. In general, the power level allowed for the SISO STX differs from the total power level allowed for the MISO STX. Our analysis shows that the relation between these power levels changes as the direct component (the K-factor) of the Ricean fading in the primary channel changes. For a large K-factor in the primary system, the total power allowed for a MISO STX is higher than the power allowed for a SISO STX system. The situation is reversed when the fading in the primary system has a low value of the K-factor and moves towards Rayleigh fading. This implies that, for example, when the direct component in the primary system is substantial, the usage of multiple antennas in the cognitive system has additional benefit, as it can use a higher power.